1. Field of the Invention
This invention relates to parallel redundant actuator systems.
2. Description of Related Art
In a parallel redundant actuator system a plurality of demand signals each indicating the desired position of an output member of the system are respectively applied to a plurality of nominally identical signal lanes whose outputs together control the position of the output member. Such a system has the advantage that, on failure of a component in one or more, but not all, of the signal lanes, the system continues to operate.
In a known form of such a system, a signal representing the actual position of the output member is fed back to the input end of each lane where it is compared with a lane demand signal, and the error signal is fed via the lane to a summing arrangement to produce an output representative of the sum of the lane output signals which is utilised to control the position of the actuator system output member.
In one particular such arrangement, the summing arrangement comprises a torque motor having a plurality of control windings to which the lane output signals are respectively applied and the motor operates a valve which controls the supply and discharge of pressurised hydraulic fluid to and from a hydraulic actuator whose output member constitutes the output member of the system.
In operation of such a system, if a failure occurs in a lane such that the signal applied by that lane to the summing arrangement no longer relates to the demand signal applied to that lane, the signals applied to the summing arrangement by the other healthy lanes change by an amount such as to compensate for the incorrect signal produced by the failed lane. However, since the output signals of the healthy lanes for a given demand signal are required to change when failure occurs in a lane, there will inevitably be a small change in the position of the actuator output member when a lane failure occurs, the magnitude of this change in position being dependent on the magnitude of the change in the output signals of the healthy lanes required to compensate for the failed lane. This problem is exacerbated where, as is commonly the case, each lane includes a further feedback loop, inside the actuator output member position feedback loop, to improve dynamic response of the actuator system.